Sahelanthropus tchadensis

Claimed as one of the most significant discoveries in the field of human evolution, the fossils possibly represent the oldest known human ancestor after the split of the human line from that of the chimpanzees. However, this is hotly debated.

Background of discovery

Age

The remains are dated to 6-7 million years old. This is a key date as it is about the time that scientists believe the human-line diverged from the ape-line.

The site lacked volcanic ash layers so was not suited to using radiometric dating techniques. Faunal analysis was used instead. This was possible because many of the fossil animals found at the site were identical to specimens that had been radiometrically dated elsewhere.

Important fossil discoveries

The discovery of six fossils (with identification numbers starting with TM 266) was made by a team, lead by Michel Brunet, between July 2001 and March 2002. The team announced the new species in 2002.

The finds included several jaw pieces, some teeth and a small but relatively complete cranium nicknamed Toumaï (‘hope of life’ in the local language). The cranium (TM 266-01-060-1) was made the type specimen. It was somewhat crushed and distorted when first discovered and some of its detail had been eroded by blowing sand.

Two other possible hominin bones (a left femur and a mandible) were found alongside these remains, as were various mammal pieces. However, they are not confirmed as belonging to the same species as the cranial remains. Taphonomic analysis reveals that the various pieces may not have been deposited at the same time, perhaps they were even transported and reburied in modern times. The femur was not recognised as possibly belonging to a hominid until 2004. As most of the diagnostic features are missing, the question of whether the femur represents a biped (or hominin) is extremely difficult.

What the name means

The genus name is made of two words. ‘Sahel’ is the area of Africa near the southern Sahara where the fossils were found and ‘anthropus’ is based on the Greek word meaning ‘man’. The species name is based on Chad, in recognition that all specimens were recovered from that country. Combined, the name means 'the Sahel man from Chad'.

Distribution

All fossils have been recovered from Toros-Menalla in the Djurab desert of Chad, Africa.

Relationships with other species

This species position is highly debated. Does it belong on our family tree or is it an ancestor of a gorilla or chimpanzee? If it does belong on our family tree, was it a direct ancestor or a distant hominin 'cousin'?

The discoverers claim that S. tchadensis has numerous derived hominin features and is therefore the oldest known human ancestor after the split of the human line from that of the chimpanzees.

If the remains are from a direct human ancestor, then the status of the australopithecine group as human ancestors is questioned. The discovers claim that S. tchadensis has advanced features, such as a thickened brow ridge, that are more similar to those of later fossil Homo and different from all australopithecines.

Others interpret the remains as being:

  • a common ancestor of humans and chimpanzees
  • related to both humans and chimpanzees, but not an ancestor of either
  • an ancestor of gorillas

Brunet disputes these interpretations and cites a study done in 2005 as conclusive evidence for his claims. In this study, CT scans were used to rebuild a more accurate picture of the skull features of Toumai. Comparisons were then done with the skulls of fossil hominins, chimpanzees and gorillas using over 30 features – Toumai fell within the hominin range for all the features.

Even if this species turns out not to be a hominin, the find would still be very significant as there are few chimpanzee or gorilla ancestors found anywhere in Africa. It would also be the best-preserved of the fossil apes of this age ever found.

More finds are needed in order to ascertain where this species fits on the human family tree. No postcranial remains have been found, so ascertaining whether it was bipedal – a key element for inclusion on the hominin branch of the tree - is difficult.

Key physical features

Brain

  • the lack of cranial remains makes estimates difficult, but brain size is estimated at about 320-380cc (similar to that of a chimpanzee)

Body size and shape

  • the lack of skeletal remains makes estimates difficult. This species was probably similar in size to modern chimpanzees.

Jaws and teeth

  • relatively small canine and incisor teeth
  • similar to Ardipithecus except for a few minor features
  • tooth enamel thickness intermediate between living apes and australopithecines
  • narrow U-shaped dental arch
  • upper and lower premolars with two roots
  • no lower jaw diastema (gap between canines and premolars)
  • cheek teeth are similar in size to Ardipithecus ramidus and Australopithecus afarensis

Skull

  • rear of the skull has an ape-like appearance
  • position of the foramen magnum suggest that it may have been bipedal, although some experts dispute this interpretation
  • scars left on the fossilised bones from its neck muscles suggest the species was a quadruped but others claim that the neck muscles attached at the back of the neck in the same way as bipeds
  • relatively flat face compared to living apes but much more protruding than modern humans
  • relatively flat nasal area
  • long and narrow base of the skull
  • large brow ridge (a presumed male trait) unlike any of those in the australopithecines
  • wider upper facial area compared to the short lower face
  • large canine fossa
  • marked post orbital constriction
  • small sagittal crest and large nuchal crest on (presumed) males
  • ape-like widely-shaped eye sockets

Limbs

  • no postcranial material has been found that has been determined beyond doubt as coming from this species

Lifestyle

Culture

No evidence of cultural attributes but this species may have used simple tools similar to those used by modern chimpanzees, including unmodified stones or sticks and other plant materials that were easily shaped.

Environment and diet

The ancient environment where the fossils were found consisted of lake, forest, river, and wooded savanna. Thousands of vertebrate fossils have also been found at the site including elephants, giraffes, antelopes, hippopotamus, crocodiles, lizards, monkeys, fish and wild boar.

It is presumed that this species was probably a plant eater.
 


Fran Dorey , Exhibition Project Coordinator
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